why is crude so funny dude?

And I have no answer for that question? Here is my question, why is it that our culture no longer laughs at clean jokes? Why do we only laugh at jokes that have sexual innuendos, foul language, and crude words? For example. I recently saw a trailer for a movie(and i still want to see it) featuring Robin Williams and John Travolta called Old Dogs. It's a comedy rated PG, so its a pretty clean, family funny movie. I looked up the reviews for this movie from the so-called 'experts', and I was astonished at what they said. Some said it was boring, others called it just not funny, in fact, one review said it was the kind of movie "you take you dumb relatives to".
On the other hand, perhaps you've heard of the movie, Its Complicated starring Alec Baldwin and Meryl Streep. Now this movie was a comedy, but rated R.The critics LOVED it, thought it was great. Now why is that? Have clean jokers like Bill Cosby given way to crude and foul comedians like Cris Rock. Why is that one cannot listen to a standup comic for five minutes without having the f-bomb thrown at you every other word?Why?Why?Why?......idk......